The star-shaped cake, whether it has five, six or more points, has enough edges zigzagging around its perimeter to give a baker pause when it's time to cover it with a layer of fondant. Cake decorating fondant, a sort of dough made primarily with icing sugar, is the element that gives professional cakes their characteristically matt, smooth look that butter cream and other frosting just can't replicate. Covering a cake with so many edges takes a little time and attention to detail, but professionals do it all the time, and an amateur can too, given a few directions.
Spread a thin layer of buttercream frosting on your star cake, making sure to cover each of the points of the star equally.
Roll out a layer of fondant large enough to fit over the entire star and cover all the points. Don't make your layer too thin. If the layer is too thin, it will likely break at the points when you start smoothing it. Also, you don't want your cake to show underneath the fondant.
Lift your fondant over your cake and drape it over the top, centred on the cake. Think of your star cake as if it were a circle as you do this to understand better how the fondant should be draped over top. Essentially, the star is just a round cake with pieces cut out.
Smooth the surface on top of the cake first, either with a fondant smoother or with your hands, and then move on to the points on the sides of your cake. Remember that the fondant is not like gift wrapping paper -- you shouldn't fold the fondant over on itself. Instead, use the flesh between your pointer finger and thumb to smooth the fondant against the cake on each point, matching the angle made by your thumb and pointer finger to the angle of the corner of the point. Work your way around the edge of the cake, smoothing and pulling, until the fondant is snug against the star.
Trim off any excess fondant from the bottom of your cake with your knife.
For sharper points, use two fondant smoothers instead of your hands. Get rid of any air bubbles with a pin. Prick the bubbles, and then resmooth the fondant.
Make sure that your coating of buttercream is completely flat to ensure a smooth layer of fondant.